My call waiting stopped working on my iPhone 7 plus. When I'm on a call I hear the beep indicating another call coming in, but no caller ID and there are no buttons to push to switch over. My phone is also not receiving calls randomly. No missed call notifications pop up or show in my call log. A voicemail will come in later if someone leaves one. How do I fix these problems? This is my work phone so I can't afford to be missing any calls.